Embodiments described and claimed herein relate to monitoring to detect clogging in a gas filter, which is utilized in a gas conduit attached to a gas blower, the latter having a motor for producing a first gas flow. Example applications include filter monitoring for Heating, Ventilation, and Air Conditioning (HVAC) systems. Such systems are used for a variety of functions, from simple ventilation to heating, cooling, and humidity control.
Generally, the purpose of a HVAC system is to move air for temperature and environmental comfort within a house, building, or factory (collectively referred to herein as building). The same purpose applies to automobiles and other structures in which healthy, comfortable air/gas quality is necessary or desired. In a HVAC system, a blower, having a motor for producing a first gas flow, moves air in the gaseous phase through one or more gas conduits (ducts) to different locations within a building before it exits the duct through a vent. After entering a duct through a HVAC system's air intake, air flows in response to a blower. Typical blowers have a motor that rotates a fan having a plurality of blades, in order to pull air through a duct in a direction that can be thought of as moving from upstream to downstream.
Many HVAC systems use filters to remove dust, dirt, contaminants, and other undesired particles that adversely affect air quality so they are not delivered past a certain location within the duct, and do not exit through a vent. Filters limit the progress of undesired particles through a duct, for example by physical restriction in which the small size of openings in a filter keeps particles from progressing through and past it, or by electrostatic attraction that hold particles to a filter.
As a filter collects undesired particles, it becomes discolored and dirty. Although a dirty filter does not substantially affect flow of air through a duct, over time, the accumulation of undesired particles produces clogging in a filter, which substantially decreases air flow in the building, and causes dust and dirt to accumulate inside the duct system and on the blower fan blades, all of which potentially reduces the quality of air in a building and adversely affects the performance of the HVAC system. Consequently, filters in systems such as HVAC systems, as well as automobiles, must be changed periodically.
Visual inspection is one way to determine when a filter should be changed, but it is time- and labor-intensive. Therefore, it is desirable in many situations to automatically sense whether a filter should be changed. This is often done by evaluating the condition of a filter as a function of measurable conditions that are related to air flowing through a duct. For example, one such air treatment system is described in U.S. Pat. No. 7,178,410, titled Clogging Detector for Air Filter, the entire disclosure of which is incorporated herein by reference. This patent discloses a filter clog detector, in which a first temperature sensor, which is coupled to a heater, and a second temperature sensor, which is unheated, are positioned in a duct. The heated sensor is kept at a temperature higher than the unheated sensor by a fixed number of degrees. If air flow velocity is greater, then more energy is dissipated away from the heated sensor. Accordingly, a measurement of the voltage required to maintain the temperature difference between the two sensors indicates the amount of air flowing through the duct where the sensors are located. Using techniques that are known to persons of ordinary skill in the art, including but not limited to those disclosed in the above-referenced patent, filter condition can be determined based on voltage readings, or similar properties having a relationship to air flow through a duct.
Other examples, regarding how filter condition is determined as a function of measurable properties or conditions within a duct, include differential pressure sensors, which respond to a pressure drop that occurs from the upstream side of a filter compared to the downstream side. Even if only to a slight degree, a new, or clean, filter restricts air flow through a duct. This restriction produces a corresponding pressure on the upstream side of the filter. Further, air passing through the filter also produces pressure on the downstream side. These pressures are measured using sensors and methods that are known to those having ordinary skill in the art. For a clean conventional filter, the differential pressure from the upstream side of the filter to the downstream side will usually be relatively small. However, as a filter becomes dirty and then clogged, the restriction on air that passes through the filter tends to decrease pressure on the upstream side. Further, because less air passes through a clogged filter, this tends to correspond to an increase in pressure on the downstream side.
Although various techniques have been disclosed for determining filter condition, as a function of measurable conditions within or related to a duct, other variables besides the extent of filter clogging may affect air flow in a duct, as indicated by measurable properties such as the voltage sensor as discussed above. For example, changes in blower speed may either increase or decrease the amount of air pulled into and through a duct. Multi-speed blowers differ from single-speed blowers and two-speed blowers, the latter of which use a two-speed motor, e.g., one speed for summer cooling and another speed for winter heating. Multi-speed blowers are capable of operating at any of a number of different speed settings. Many blowers require a period of time to stabilize and reach a consistent operating speed. For example, some blowers require approximately 60 seconds for this to occur.
Consequently, when determining when a filter should be changed, it is desirable to ascertain that readings of measurable conditions in a duct are actually indicative of a change in filter condition over time. In other words, it is desirable to prevent there from being an indication of clogging merely because of changes in blower speed.